The trend of development shows that networks and services will be developed over the Internet Protocol (IP). A key technology for the evolution of conventional networks based on Synchronous Digital Hierarchy (SDH) to IP-based packet transport networks is to solve the clock and time synchronization in IP packet networks. The conventional Second Generation (2G) wireless telephone technology only requires frequency synchronization and generally obtains a clock synchronization signal through a Time Division Multiplexing (TDM) network of, for example, SDH and Plesiochronous Digital Hierarchy (PDH). In scenarios where the synchronization signal cannot be obtained from the TDM network (PDH or SDH), a Global Positioning System (GPS) is generally used to provide a high-precision synchronous clock reference source. A 3rd Generation (3G) network requires not only a frequency synchronization precision of 500 parts-per-billion (500 ppb) but also a time synchronization precision measured by microseconds. In a Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) network, for example, the required precision of time synchronization is within ±1.5 microseconds. The conventional PDH and SDH networks only provide frequency synchronization. The Network Time Protocol (NTP) technology only provides a time synchronization precision measured by milliseconds. These obviously do not meet the technical requirements of 3G networks. In addition, as the investment in network infrastructure gradually drops, the expensive GPS service is not a best solution. Therefore, the Precision Time Protocol (PTP) gradually becomes the mainstream time synchronization standard in the industry.
According to the PTP protocol, timing messages are exchanged between master and slave devices to implement time synchronization. If asymmetric delays exist in the sending and receiving communication paths, the synchronization precision becomes worse because of the asymmetric delays. In the telecommunication field, most communication paths are connected through optional fibers. In engineering, it is hard to keep the sending and receiving fibers strictly symmetric.
The delay caused by a 1-meter fiber is about 4.86 ns. According to the PTP protocol, the synchronization error equals ½ of the asymmetric delay of the sending and receiving fibers. Therefore, the asymmetry of a 500 m fiber will cause a time synchronization error of about 1,215 ns. This is already likely to affect the time synchronization in a 3G network.
Although the PTP protocol defines the asymmetric model of the communication path delay and provides a method for compensation in a Correction Field, the protocol does not define how to measure the asymmetry of such transmission delays. Wireless technologies require a very high time synchronization precision. The delay error caused by the asymmetry of fibers has a great adverse impact on the time synchronization precision of PTP. Therefore, the asymmetry of fibers must be considered in engineering implementation. Most fibers were laid years ago when the asymmetry of filers was generally not considered. Even in the case of new fibers, it is still hard to keep the fibers symmetric in terms of engineering. If the asymmetry of fibers cannot be solved, the large-scale commercial use of PTP will be seriously affected.
A method for measuring the delay caused by the asymmetry of communication paths in a prior art uses the GPS or an Optical Time Domain Reflectometer (OTDR) to perform measurements point by point and compensates the asymmetry according to the measurement results. This method requires an on-site measurement at every site. The workload is huge. Moreover, when the GPS is used for the measurement, satellites must be within the vision of the receiver. If a NodeB is deployed in a basement or subway station where GPS antennas are hard to install, the implementation is very difficult and the cost is high. The operations on an OTDR are complicated and require high skills of the operator. With the above measurement method, when a broken fiber is reconnected through welding, the physical length of the fiber is changed and a new on-site measurement is required. The repair of a broken fiber is therefore complicated and costly.